High-level radioactive wastes included in radioactive wastes yielded from nuclear power generation include liquid wastes separately obtained by spent nuclear fuel reprocessing. More specifically, the high-level radioactive wastes show a high radioactivity level, and besides, contain a large number of radioactive nuclides having so extended life as to continue to hold radioactivity over a long period of time. For that purpose, such high-level radioactive wastes are given stabilizing by being processed into vitrified wastes in such a manner as to pour these radioactive wastes in a molten state into stainless steel canisters together with glass materials, followed by being reserved for several ten years for the sake of cooling. Afterwards, the canisters containing the vitrified wastes are received etc. in an airtight state into thick steel plate-made airtight containers called overpack to provide waste matters, causing the waste matters to be buried in stable underground stratums having a depth of 300 m or more (specified in the law).
This type of geological disposals of the radioactive wastes is supposed to take measures to guarantee the safety with a multiple barrier system constructed by a combination of artificial barriers including bentonite buffer materials around the waste matters with natural barriers including bedrock. The artificial barriers serve to reduce a rate of emission of nuclides from the waste matters to the natural barriers. The natural barriers serve to retard migration of the nuclides toward the biosphere.
The bentonite is a general term of a group of resource minerals containing, as main components, montmorillonite included in clay minerals, and, as coexistent mineral components, minerals such as quartz, calcite and plagioclase. The montmorillonite is in the form of thin planar crystals (about 0.2 μm in length), and presents impermeability because application of pressures by processing such as compaction after swelling by absorption of water into inter-crystal voids brings about stratification. The montmorillonite also may prevent interlayer water from being migrated because water molecules suffer ion-mannered attraction in electric double layers, causing inter-layer clearances to be narrowed under pressures. For the above reasons, the bentonite is used as impermeable materials. As for the geological disposals of the radioactive wastes, use of the bentonite is made from the viewpoint of its performances such as impermeability with respect to underground water, buffering with respect to bedrock pressures and retardation of radioactive nuclide migrations.
In the existing idea of the geological disposals of the radioactive wastes, the bentonite-contained artificial barriers are schemed in such a manner as to be constructed by giving compaction with a heavy construction machine to, or static compression to the bentonite having undergone adjustment to a prescribed moisture content. It is indefinitely supposed that moisture content adjustment in this case is made by means of water adding in advance of sufficient stirring and mixing or by means of water sprinkling.
By the way, Patent documents 1 to 6 are in existence as prior art literatures related to the present invention. Inventions described in Patent documents 1 and 2 are those relating to a bentonite preparation method. Inventions described in Patent Documents 3 to 6 are those relating to a concrete preparation method of kneading concrete using small cakes of ice as a substitute for water.    [Patent Document 1] Japanese Patent Laid-open Hei 8-277108    [Patent Document 2] Japanese Patent Laid-open Hei 6-293512    [Patent Document 3] Japanese Patent Laid-open 2002-144325    [Patent Document 4] Japanese Patent Laid-open 2002-11709    [Patent Document 5] Japanese Patent Laid-open 2001-293718    [Patent Document 6] Japanese Patent Laid-open Hei 6-179209
The moisture content adjustment of bentonite is indefinitely supposed to be made by means of water adding in advance of sufficient stirring and mixing or by means of water sprinkling, in which case, however, an attempt to merely add water to the pulverulent bentonite or sprinkle the pulverulent bentonite with water causes only the bentonite in contact with water to be turned into granular lumps having high moisture contents, resulting in a remarkable lack in uniformity. Further, a powerful mixer is required to stir and mix hydrous bentonite until its uniformity is obtained, and besides, it is necessary to stop the mixer several times during stirring and mixing in order to remove the bentonite cohered to a mixing blade and/or a mixing tank. For this reason, an attempt to make the moisture content adjustment of a huge quantity of bentonite required for construction of the bentonite-contained artificial barriers is at variance with the reality. In addition, stirring and mixing in a continuous manner have been impossible of attainment.
Further, the bentonite having undergone the moisture content adjustment with a conventional method is turned into the granular lumps (of large grain size), causing a bentonite condition immediately after compaction to go into “raising”. Thus, when the moisture content-adjusted bentonite obtained with the conventional method is compacted for the sake of use as a cut-off material, no cut-off performance could be expected before the bentonite in the form of granular lumps so swells in association with seepage as to fill up voids. Furthermore, the moisture content-adjusted bentonite obtained with the conventional method is lacking in uniformity of moisture content distribution, so that drying of the bentonite of this type causes remarkable shrinkage of its high moisture content portions, resulting in creation of a large number of great cracks.
The present invention is intended to provide a material moisture content adjustment method, which is adaptable, when making moisture content adjustment by adding liquid such as water to a raw material such as pulverulent material including bentonite, to uniformly mix the liquid such as water with the raw material such as pulverulent material using relatively simple facilities, also to easily attain moisture content adjustment of a large quantity of raw materials, and further to obtain a material having satisfactory performances such as impermeability through uniform moisture content adjustment.